Effect of structural and optical properties on photocatalytic performance of Sr-doped ZnO nanoparticles for removal of organic pollutants and its mechanism pathway

Sr-doped ZnO nanoparticle (ZnO NP) photocatalysts were synthesized from a PEG600-modified precursor solution via a facile precipitation method. A calcination temperature of 600 °C was determined by thermal analysis. The structural and chemical properties of the calcined photocatalyst were characterized by various techniques, which confirmed the successful formation of Sr-doped ZnO NPs. Optical properties of the ZnO and Sr-doped ZnO photocatalysts were characterized by UV–Vis diffuse reflectance spectrophotometry. The optical bandgap energy of Sr-doped ZnO diminished from 3.155 to 3.137 eV as a function of Sr²⁺ ion concentration. The 5 mol%Sr-doped ZnO NPs outperformed other photocatalysts, degrading 100% of methylene blue (MB) solution within 150 min under UV irradiation. The pseudo-first-order kinetic rate constant of the MB degradation over 5 mol%Sr-doped ZnO NPs was 0.0245 min⁻¹. The performance of the optimal photocatalyst was assigned to its larger specific surface area (3.748 cm²/g) and total pore volume (0.0706 cm³/g). ^•OH radicals and h⁺ species were key active species in the degradation reactions. The fragments of degraded MB were determined by liquid chromatograph-quadrupole time-of-flight mass spectrometry, and the mechanism of photocatalytic degradation was proposed. The 5 mol%Sr-doped ZnO NPs degraded MB faster than methyl orange, and could be an appropriate photocatalyst for industrial use because of its efficiency, stability, and sustainability.